U.S. Pat. Nos. 4,531,857 and 4,531,859 disclose a revolutionary new design for roadway decks. These patents disclose a prefabricated pavement module for bridges and the like and a method for making the module. These deck modules are generally known in the industry as exodermic decks. Exodermic decks may be used to replace worn out or damaged conventional decks as well as to construct new roadways.
The exodermic deck is a design concept that combines a steel grid and reinforced concrete in a unique way. It maximizes the use of the compressive strength of concrete and the tensile strength of steel. Based on working stress principles, the design positions stress raisers in the steel grid at or close to the neutral axis of the composite deck.
The known exodermic deck includes a reinforced concrete component on top of, and bonded to, a welded steel grid or grating component typically including primary load bearing bars, secondary load bearing bars, and tertiary load bearing bars. The dimensions and properties of each component of the deck are selected for the specific bridge by the design engineer. The design is composite within itself and can be made composite with most types of existing or new bridge framing systems. In a typical practical application, the concrete component embeds a two-way web of epoxy-coated reinforcing bars. Vertical studs welded to the tertiary load bearing bars of the steel grid are also embedded in the concrete component of the deck. Horizontal shear transfer is developed through partial embedding of the tertiary load bearing bars in conjunction with the vertical studs.
An exodermic deck has section properties increased by 150% to 300% over that of known conventional grid deck constructions. High load capacity and extended useful life are provided by relocating the neutral axis of the composite deck, which reduces the maximum stress level in the top surface of the grid to a point at which fatigue failure should not occur. The exodermic deck system also eliminates the need for constant repair of broken grid bars and connections which is common in open grid deck installations. Moreover, an exodermic deck eliminates skidding and noise problems commonly associated with open grid deck bridges and with filled grid deck bridges which do not have a wearing surface above the grid. The exodermic deck also is significantly lighter than known filled or partially filled grid decks. This is highly desirable in bridge construction.
However, both with conventional open grid decks and filled grid decks, and with exodermic grid decks, known methods of making the heavy steel grids conventionally used structural welds to hold the primary, secondary, and tertiary load bearing bars together. The structural welding processes require the use of complex, fixed jigs to hold the load bearing bars in proper position. The jigs must clamp the bearing bars in a complex convex configuration so that after welding, when the welds and bars cool and contract, the resulting grid will be flat.
Another disadvantage of known construction methods for grid decks is that only rectangular grid modules have been formed. However, most bridges are skewed, having a nonrectangular parallelogram shape rather than a rectangular shape. Thus, it is highly desirable to produce bridge deck modules in the shape of nonrectangular parallelograms to match the bridge skew. In order to match the skewed bridge structures, present methods require the formation of a rectangular grid along with the formation of separate triangular grid pieces which are welded to the ends of the rectangular grid. This is a costly and complicated process.